The present invention relates to a color Braun tube with an in-line type electron gun, which produces a high-definition picture display.
An in-line type color Braun tube may not encounter a severe problem when it is used as a color television picture tube to receive pictures sent by a standard broadcasting method. However, if an in-line type color Braun tube is used as a monitor for a computer, requiring high-definition performance, since many scanning lines have to be produced at a high frequency in such a monitor, a problem occurs in that a large misconvergence is caused between the scanning area, namely, between the image effective area irradiated by the central beam of the three beams aligned in the horizontal direction, and the image effective areas irradiated by the two beams at both sides, during high frequency beam scanning.
A main cause of the problem can be explained as follows. A shield cup electrode, made of a non-magnetic metal for use in an in-line color Braun tube, is composed of a conductive cylindrical side shield wall surrounding the three beams, and a base plate arranged to face the cathode of the tube and in which three beam passing holes are provided. Further, the shield cup electrode is arranged at the end of the electron gun for generating the three beams aligned in the horizontal direction so as to face the fluorescent screen of the tube, in order to shield the beams from the influences of an electrostatic charge accumulated at the inner surface of the glass bulb of the tube. A deflection yoke for generating a deflection field to deflect the beams is arranged on the outside of the glass bulb where the neck of the tube joins the funnel part in the tube, so that a part of the deflection field, nearer to the cathode, passes the side wall of the shield cup electrode. Therefore, eddy currents are induced in the conductive side wall by the momentarily changing deflection field, and the induced eddy currents act to weaken the deflection field generated by the deflection yoke. In the case of a low deflection frequency such as used in the standard broadcasting method, the influence of the eddy currents on the deflection field is negligible, since the misconvergence is small, even if the image effective area irradiated by the central beam and by both side beams do not converge into one area. On the other hand, in a display tube with high-definition performance of the type used for a monitor of a computer, since the number of scanning lines and the time change rate of the horizontal deflection field are considerably larger than those of a display tube used for a standard broadcasting method, the eddy currents induced in the side wall of the shield cup electrode becomes much larger and remarkably affects the deflection field.
FIG. 11A and FIG. 11B illustrate the structure of a shield cup electrode of an electron gun such as used in the in-line type color Braun tube disclosed in JP-A-190232/1988. As shown in the figures, three beam passing holes 4, 5 and 6 are provided in a horizontal line in a base plate of the shield cup electrode 1 for shielding the beams from the influences of an electrostatic charge accumulated at the surface of the glass bulb of the tube, and the three beams generated by the electron gun are passed through the holes and formed as three horizontally parallel beams. At the upper and lower portions of each of the side holes 4 and 6 of the beam passing holes 4, 5 and 6, a pair of projecting plates 20a are provided by bending a pair of rectangular plates projecting from a non-magnetic metal base member 20, so that they project perpendicularly from the base member 20 attached at the base surface 1b of the base plate 1c in parallel to each other. The non-magnetic metal base member 20, having two pairs of the bent projecting plates 20a, is welded at the points 3 between the hole 4 and the hole 5 and between the hole 5 and the hole 6, in an area of the base member 20 disposed between the two pairs of the bent projecting plates, respectively. The two welded points 3 are indicated with a mark x.
Further, JP-A-190232/1988 describes the effects of the above-mentioned structure of the shield cup electrode as follows. That is, the force of the horizontal deflection field is equally applied to each of the three beams aligned in the horizontal direction, due to influences of eddy currents induced in the two pairs of bent projecting plates 20a of the non-magnetic metal base member 20. Thus, even with a high frequency deflection field, any misconvergence due to eddy currents flowing in the shield cup electrode is suppressed to a negligible level.
Color Braun tubes having a similar structure are disclosed in JP-A-181637/1992 and JP-A-249040/1992, respectively. In the tube disclosed in JP-A-181637/1992, step-wise members corresponding to the above-mentioned bent projecting plates 20a are provided by using annular magnetic field shielding elements made of high-permeability material, and further slits are provided at each of the step-wise members. The use of high-permeability material is effective to shield the beams from the outer magnetic field. Furthermore, the shape of the step-wise members is also effective to suppress eddy currents induced by the high frequency deflection field. In the tube disclosed in JP-A-249040/1992, each of the annular magnetic field shielding elements made of high permeability material, corresponding to the above-mentioned bent projecting plates 20a, is accurately positioned by using a circular arc shape projecting rim. Also in this case, the use of high-permeability material is effective to prevent chromatic aberration. Furthermore, the circular arc shape projecting rims are used to suppress eddy currents induced by the high frequency deflection field.
Another cause of the misconvergence between the image effective areas irradiated by the central beam and the two beams on either side can be explained as follows.
That is, a series of non-magnetic metal electrodes forming electron lenses for condensing each of the beams on a fluorescent surface of the tube are arranged between the cathode generating the beams in the tube and the non-magnetic metal shield cup electrode. Since eddy currents induced in the conductive side wall of the shield cup electrode by a changing deflection field generated by the deflection yoke flows into the electron lens forming electrode adjacent the shield cup electrode, eddy currents are consequently generated in a wide region of the shield cup electrode and the electron lens forming electrode adjacent the shield cup electrode. The eddy currents generated in a wide region weaken the deflection field generated by the deflection yoke. As the time change rate of the horizontal deflection field becomes larger, the eddy currents become much larger and more strongly affect the deflecting field. However, a technique for suppressing the misconvergence caused by the eddy currents to an acceptable level, taking also the other above-mentioned cause into account, has not been devised yet.